The Shadow-Mask Colour CRT

DISCHARGE FIRST, AND ASSUME IT IS CHARGED. The anode connector under the rubber cup on the side of the bell is the top of a capacitor formed by the glass with conductive coatings on both faces. It holds tens of kilovolts and it holds them for weeks. It will also RE-CHARGE ITSELF after a first discharge as the glass depolarises, which is why one discharge is not enough. Clip an earth lead to the tube's external dag coating or to the metal band around the rim. Take an insulated screwdriver with a long shaft, connect the earth lead to its shank through a 1 megohm resistor if you have one, slide the blade under the anode cup, and touch the connector. Expect a crack. Leave it shorted for a minute, remove it, wait five minutes, and DO IT AGAIN. Do it a third time before you put a hand anywhere near it. IMPLOSION. The envelope is at vacuum and it is large. Modern tubes have a tensioned steel band bonded round the rim specifically to make them fail by cracking rather than by imploding, but do not test it. Wear a full face shield and heavy gloves, keep the tube face-down on a blanket, and never lift or carry it by the neck — the neck is thin, and a snapped neck vents the tube instantly. If you are not going to open one, everything below can be done on an intact tube through the faceplate with a loupe. That is the recommended route, and you lose only the mask temperature measurement. FIND THE MASK. Look into the face of an unpowered colour tube from a few centimetres with a bright torch at a shallow angle and a 10x loupe. You will see the triads: three round dots in a triangle, or on a later in-line tube three vertical stripes. Behind them, slightly out of focus, is the mask. Rock your head side to side and the mask holes move against the dots — that parallax is the whole mechanism made visible, and it is the clearest demonstration in this blueprint. MEASURE THE PITCH. Put a steel rule against the faceplate and count triads over 10 mm with the loupe, or photograph the screen through a macro lens with the rule in frame and count in software. Domestic tubes ran a dot pitch between about 0.25 mm and 0.8 mm depending on size and era; computer monitors went finer, to 0.21 mm in the best of them. Record the number, because the next step needs it. MEASURE THE OPEN AREA. On a removed mask, backlight it and photograph it against a scale. Measure a hole diameter and the hole spacing. Open area is the total hole area over the total sheet area, and it will come out somewhere between fifteen and twenty percent. On an intact tube, take the manufacturer's figure of 0.18 and note that you assumed it. CONFIRM THE MASK IS STEEL AND WHAT THAT COSTS. Bring a small magnet near the faceplate of a POWERED tube showing a white field, from 100 mm away, and move it slowly. The colour will bloom and tear into blotches of red and green and blue. You have deflected the beams before the mask so they now pass through the holes at the wrong angles and land on the wrong dots. Remove the magnet: the blotches STAY. The mask itself is now magnetised. That is why every colour set has a degaussing coil round the face and a positive-temperature-coefficient thermistor that drives a decaying alternating current through it at every switch-on. A shadow mask is a large piece of low-carbon steel sitting in the Earth's field, and simply turning a television to face a different direction is enough to shift the beams. Degauss it by switching the set off and on a few times with a minute between, and the picture will come back.